1. Technical Field
The present disclosure relates to silicon carbide semiconductor substrates.
2. Description of the Related Art
Wide bandgap semiconductors are used in various semiconductor devices such as power devices (power elements), environment resistant devices, high-temperature devices and high-frequency devices. In particular, the application of attention is to power devices such as switching devices and rectifying devices.
Of the wide bandgap semiconductors, silicon carbide (SiC) is relatively easy to form into substrates. Further; the thermal oxidation of this semiconductor material gives silicon dioxide (SiO2) films that are quality gate insulators. These benefits make SiC attractive for use in the development of power devices (see, for example, Japanese Unexamined Patent Application Publication No. 2012-151400).
Some of the typical switching devices using SiC are metal insulator semiconductor field effect transistors (hereinafter, “MISFETs”) and metal semiconductor field effect transistors (hereinafter, “MESFETs”).
SiC has a higher dielectric breakdown field and a higher thermal conductivity than Si. Thus, power devices using SiC (SiC power devices) can achieve a higher breakdown voltage and a lower power loss than Si power devices. This fact makes it possible to significantly reduce the area and thickness of the SiC power devices as compared to Si power devices having an equal performance, resulting in a decrease in gate electrode-to-substrate parasitic capacitance. Further, SiC has a higher electron saturation speed than Si. These characteristics allow the SiC power devices to be switched at a much higher speed than Si power devices.
Because the coefficient of thermal diffusivity of impurities in SiC is low, it is difficult to control the diffusion of impurities by a thermal method used in other semiconductors such as Si. While ion implantation is used to form relatively shallow impurity layers, gas-phase doping in which a dopant is added during the epitaxial growth of crystal is effective to control the carrier concentration in deep impurity layers such as drift layers in vertical MISFETs. This gas-phase doping is generally performed by a chemical vapor deposition (CVD) method. The CVD method is useful in that the impurity concentration and other properties such as pn junction interface can be controlled and also in that the method can be applied not only to small substrates but also to large substrates.
SiC crystal is conventionally grown with a horizontal CVD apparatus such as one described in Japanese Unexamined Patent Application Publication No. 2010-40607. However, as reported by Xuan Zhang, et al, in Materials Science Forum, Vols. 679-680 (2011), pp. 306-309, the temperature distribution in a wafer gives rise to the occurrence of interfacial dislocations during the epitaxial growth of SiC.